1. Field of the Invention
The present invention generally relates to the output stage of a circuit, and more particularly to a slew rate control circuit for a high-speed low-power transmitter.
2. Description of the Prior Art
A Mobile Industry Processor Interface (MIPI) is a high-speed low-power (LP) serial transceiver interface developed to support interconnections of low-power high-speed mobile applications such as for digital camera, display or other portable devices. A D-PHY specification further defines physical layer devices that transport high-speed data on the link between the transmitters and the receivers.
The MIPI specification regulates, among other things, the slew rate and the output impedance (Rout) of a transmitter (Tx).
The slew rate is the maximum rate of change of a signal in a circuit. Low slew rate causes signal distortion. On the other hand, the output impedance of a transmitter should be as large as possible such that the output of the transmitter may not be affected by an external load. In general, for designing a practical circuit, such as the MIPI low-power transmitter (LP Tx) mentioned above, a compromise between the slew rate and the output impedance should be reached. According to the MIPI specification, the slew rate of a transmitter with a load of capacitance of 0-70 pF should be within a specific range, and the output impedance should be not less than a specific value. In order to be in conformity with both the slew rate and the output impedance, the design of a MIPI low-power transmitter thus becomes complex and costly.
FIG. 1 shows a schematic circuit illustrating the output stage of a MIPI transmitter. In the figure, a capacitor 10 and a resistor 12 are connected between the output node and the input node as shown to compromisingly arrive at an acceptable slew rate and output impedance. Unfortunately, when the external load increases, the transmitter may necessarily be designed to increase its output to maintain the slew rate, while disadvantageously decreasing its output impedance at the same time. In other words, the slew rate and the output impedance usually affect each other in a manner that a designer would prefer mostly to avoid.
For the reason that conventional circuit, such as a MIPI transmitter, could not be effectively designed without using complex circuitry to reach an acceptable tradeoff between the slew rate and the output impedance, a need thus has arisen to propose a high-speed interface circuit that can take account of the slew rate and the output impedance at the same time.